OFDM, OFDA, and SC-FDMA methods that are used in the present invention will now be described in detail.
The demand for high speed data transmission has recently been increasing, and, being advantageous in high speed transmission, the OFDM method has been selected as an adequate transmission method in various types of high speed transmission systems.
Hereinafter, OFDM (orthogonal frequency division multiplexing) will be described in detail. The basic principles of OFDM corresponds to dividing a high-rate data stream into a plurality of slow-rate data streams, thereby simultaneously transmitting the slow-rate data streams by using a plurality of carrier waves. Each of the carrier waves is referred to as a subcarrier. Due to an orthogonality existing between each carrier wave of the OFDM method, a receiving end may detect a carrier wave frequency element even when the carrier wave frequency elements are overlapped with one another. The high-rate data stream passes through a serial to parallel converter so as to be converted into a plurality of slow-rate data streams. Then, a subcarrier is multiplied to each of the converted data streams. Subsequently, each of the data streams is added and then transmitted to the receiving end. The OFDMA corresponds to a multiple access method allocating subcarriers to an entire band in accordance with a transmission rate required by multiple users in the above-described OFDM.
Hereinafter, a related art SC-FDMA (Single Carrier-FDMA) method will be described. The SC-FDMA method is also referred to as a DFT-S-OFDM method. The related art SC-FDMA method is a method that is generally applied to uplinks. Prior to generating an OFDM signal, the related art SC-OFDM method adopts a process of spreading symbols by using a DFT matrix in a frequency domain. Thereafter, the result of the spreading process is demodulated by using the related art OFDM method, thereby being transmitted. The following variables will be defined in order to describe the SC-FDMA method. N represents a number of subcarriers transmitting an OFDM signal, and Nb indicates a number of subcarriers for an arbitrary user. F signifies a discrete fourier transform matrix, i.e., DFT matrix. s indicates a data symbol vector, x represents a vector having data dispersed in the frequency domain, and y signifies an OFDM symbol vector being transmitted in a time domain.
In the SC-FDMA method, a data symbol (s) is dispersed by using the DFT matrix before being transmitted. This process is represented by Equation 1 shown below.x=FNb×NbS  [Equation 1]
In Equation 1, FNb×Nb represents a DFT matrix having the size of Nb and used for dispersing the data symbol (s). A subcarrier mapping process is performed on the vector (x), which is dispersed as described above, by using a method of equally allocating subcarriers, thereby converting the vector (x) to a time domain by using an IDFT module, thereby obtaining a signal that is to be transmitted to a receiving end. The transmission signal that is transmitted to the receiving end is represented by Equation 1 shown below.y=F−1N×Nx  [Equation 2]
$In Equation 2, F−1N×N represents a DFT matrix having the size of N and used for converting a frequency domain signal to a time domain signal. A cyclic prefix is inserted in the signal y and then transmitted, the signal y being generated by using the above-described method. The method of generating a transmission signal by using the above-described method and then transmitting the generated signal to the receiving end is referred to as the SC-FDMA method. Herein, the size of the DFT matrix may be diversely controlled with respect to a plurality of specific purposes.
The description presented above is based upon a DFT or IDFT calculation. However, for simplicity, in the following description, a DFT (Discrete Fourier Transform) or FFT (Fast Fourier Transform) calculation will be used without any particular indication. If the number of input values of the DFT calculation is equal to a power of 2, it is apparent to those skilled in the art that the FFT calculation may be used instead of the DFT calculation. Therefore, in the following description, the contents related to the FFT calculation may also be equally applied to the DFT calculation.
Hereinafter, a sequence used in a 3GPP (3rd Generation Partnership Project) LTS (Long Term Evolution) technology, which has recently been proposed as a new technology, will be described. A wide range of sequences is also used in the LTE system. Hereinafter, a sequence used in a channel of the LTE system will now be described. Generally, in order to communicate with a base station, a terminal first performs synchronization with the base station through a synchronization channel (hereinafter referred to as ‘SCH’) and then performs cell search.
A series of process for performing synchronization with the base station and acquiring a cell ID of the corresponding terminal is referred to as a cell search. Generally, the cell search may be divided into an initial cell search which is performed when an initial terminal has turned its power on, and a neighbor cell search which performed for searching a neighboring base station of a terminal in a connection or idle mode.
The SCH (Synchronization Channel) may be configured to have a hierarchical structure. For example, a P-SCH (Primary-SCH) and a S-SCH (Secondary-SCH) may be used. Herein, the P-SCH and the S-SCH may be included in a radio frame by using diverse methods. FIG. 1 and FIG. 2 illustrate a plurality of methods by which the P-SCH and the S-SCH are included in a radio frame. In the LTS system, depending upon various circumstances, the SCH may be configured in accordance with the structures shown in FIG. 1 and FIG. 2.
Referring to FIG. 1, the P-SCH is included in the last OFDM symbol of the first sub-frame. And, the S-SCH is included in the last OFDM symbol of a second sub-frame. Meanwhile, referring to FIG. 2, the P-SCH is included in the last OFDM symbol of the first sub-frame. And, the S-SCH is included in the second last OFDM symbol of the first sub-frame.
The LTE system may use the P-SCH to acquire time and frequency synchronization. Additionally, a cell group ID, a frame synchronization information, an antenna configuration information, and so on, may be included in the S-SCH. Hereinafter, the method of configuring the S-SCH proposed in the related art 3GPP LTE system will now be described in detail.
Referring to FIG. 1 and FIG. 2, two S-SCHs are included in one radio frame, and, preferably, each of the two S-SCHs corresponds to a different sequence. Also, it is preferable that the amount of information that is to be included in a S-SCH is equal to 1020 units (or types). More specifically, a 1-bit information for frame synchronization (i.e., frame synch), a 8-bit information representing the cell group ID, and a 2-bit information indicating a transmission antennae through which signals are being transmitted are included in the S-SCH. 2 different types of 1-bit information, 170 different types of 8-bit information, and 3 different types of 2-bit information may be indicated. In other words, 2*170*3=1020 different types of information may be indicated.
Although, the example of the particular number of information sets included in the S-SCH has been proposed to be equal to 1020, the description does not suggest or propose in detail as to how the information will be represented. Hereinafter, the synchronization channel of an IEEE 802.16e system will now be described in detail. When using the OFDMA-based IEEE 802.16e system, a preamble configured of one OFDM symbol is first transmitted for each downlink frame. The preamble is provided to a telecommunication terminal for diverse purposes such as synchronization, cell search, and channel estimation in a telecommunication system.
FIG. 3 illustrates a structure of a downlink sub frame in the IEEE 802.16 system. Referring to FIG. 3, the preamble, which is configured of one OFDM symbol, is transmitted in each frame preceding the other signals. Herein, the preamble is used for diverse purposes, such as time and frame synchronization, cell search, channel estimation, and so on.
FIG. 4 illustrates a group of subcarriers transmitting a preamble, which is being transmitted from a 0th sector, in the IEEE 802.16 system. Herein, part of both ends of a given bandwidth is used as a guard band. In addition, when the number of sector is equal to 3, each sector inserts a sequence for each 3 subcarriers, and 0 is inserted in the remaining subcarriers.
Hereinafter, a related art sequence used in the preamble will now be described. The sequence being used in the preamble is shown in Table 1 below.
TABLE 1IndexIDcellSectorSequence (Hexadecimal numbers)000A6F294537B285E1844677D133E4D53CCB1F182DE00489E53E6B6E77065C7EE7D0ADBEAF110668321CBBE7F462E6C2A07E8BBDA2C7F7946D5F69E35AC8ACF7D64AB4A33C467001F3B22201C75D30B2DF72CEC9117A0BD8EAF8E0502461FC07456AC906ADE03E9B5AB5E1D3F98C6E. . .. . .. . .. . .
Herein, the sequence is decided by the sector number and the IDcell parameter value. Each of the defined sequences is converted to a binary signal in an increasing order, so as to be mapped to the subcarrier by using BPSK modulation. In other words, the proposed hexadecimal number sequence is converted to a binary number sequence (Wk). Then, the Wk is mapped from the MSB (Most Significant Bit) to the LSB (Leas Significant Bit). At this point, 0 is mapped to +1, and 1 is mapped to −1. (For example, in a 0th segment having an index of 0, since the Wk corresponding to the hexadecimal number ‘C12’ is 110000010010 . . . , the converted binary code value becomes −1 −1 +1 +1 +1 +1 +1 −1 +1 +1 −1 +1 . . . )
The sequence of the related art corresponds to a sequence searched by a computer simulation. This sequence corresponds to a sequence among a plurality of sequence types that may be configured by using binary codes, the being capable of comparatively maintaining Correlation characteristics and, at the same time, maintaining a PAPR (Peak-to-Average Power Ratio) at a low level when being converted to the time domain.
Meanwhile, in a more evolved (or upgraded) system type, such as an IEEE 802.16m system, the synchronization channel identical to that of the IEEE 802.16e system may be applied herein, and the purposes may also be the same. However, details as to how to represent the synchronization channel remain undefined in such upgraded version of the IEEE 802.16 system as well.